The present invention relates to a solar cell manufacture technique. More particularly, the invention is objected to provide an electrically conductive paste designated for forming front electrode of solar cells and methods for manufacturing the conductive paste.
Solar energy is an inexhaustible source of clean energy. With the increasing depletion of coal, oil and other non-renewable energy, development and use of solar energy have become a big trend of exploring renewable energy. The use of solar cells is a typical means of collecting solar energy, wherein the solar cell made by crystalline silicon is currently a major solar cell technology and will be in the market for substantially long time even though new generation thin-film solar cell has also been developed.
Crystalline silicon solar cells are in general composed of a front side electrode, an anti-reflective coating, an emitter, a P—N junction, a base, an aluminum back surface field, and a back electrode. The front side electrode collects photon-generated charge carriers near the front side electrode and supplies current.
The front electrode in crystalline silicon solar cells of the prior art is made from a conductive paste composed of silver powder, glass frit, one or more additives, and an organic carrier. Usually a glass frit in the electrically conductive paste has the following effects: a) wetting the metal powder to promote the sintering of the metal powder; b) etching the antireflective coating layer which is an insulating layer (e.g., silicon nitride) to promote the contact between the sintered metal and the silicon-based (n-type semiconductor) material. In order to achieve a good ohmic contact, the antireflective coating layer must be etched through but free from penetrating into the PN junction region of the silicon-based material.
The choice of the glass frit, its composition, softening point, thermal expansion coefficient, wetting properties, and amount (within the conductive paste), etc. will affect the physical and chemical changes of the conductive paste in the sintering process to form the electrode, thereby affecting the solar cell performance. In the sintering process, the glass frit material is gradually softened. Within a short process cycle, usually 1-2 minutes, part of the softened glass frit remains around the metal powder while another part of the softened glass frit material flows to reach the antireflective coating layer at bottom and induce an etching reaction there. On one aspect, the amount of the glass frit is an important factor affecting quality of the electrode, which also causes many physical constraints to the manufacture process. If the amount of the glass frit is not enough, there is no sufficient contact formed between the softened glass frit material and the antireflective coating layer to ensure that the antireflective layer is completely penetrated. If the glass frit is added too much, the probability of mutual contact between the conductive silver powders is too low so that the conductive phase among the as-formed electrode material becomes too scarce, causing the conduction performance of the front electrode of solar cell severely deteriorated. On another aspect, the glass frit is engineered through the selection of glass materials with a lower softening point, such as <400° C., in order to ensure that a sufficient amount of glass frit is deposited on the surface of the antireflective layer in the entire process, and therewith complete the etch reaction to remove the antireflective coating. But premature softening of the glass frit, can clog the communicating channels between the metal powders, and hinder the effective discharge of the organic carrier.
Pb—Si based glass materials usually are chosen for the manufacture of the front electrode paste. More recently, Pb—Te oxide and other oxide materials or fluoride materials are chosen to go through a series of processes of melting, mixing, and quenching the molten mixture to form a glass material before milling the glass material into the glass frit. However, regardless the use of various alternative materials, the nature of the glass material by itself sets many physical performance and chemical reaction constraints such as a narrow sintering process window for transforming the printed conductive paste to the electrode with desirable electrical conductivity while preventing from the emitter being penetrated. Presently, most of the conductive pastes on sale for manufacturing the front side electrodes of crystalline silicon based solar cells have these technical limitations.